Quasar Redshift vs. Discovery Date

This plot displays the logarithm (base 10) of the number of catalogued
quasars as a function of time. By the end of 1963 a handful of quasars
were known. Early discoveries of quasars relied on their radio emission.
The rapid increase in the number of quasars in the 1970s is due to their
being identified by their optical colors. The SDSS is using the same
technique to find quasars.
Image credit: Don Schneider, SDSS Collaboration